They've sparked riots from Boston to Milan, sold out concerts from Hong Kong to Hamburg. Each of their five previous albums has gone platinum, selling more than one million copies; one, Led Zeppelin (IV), has sold more than three million. They've set new records for U.S. concert attendance, drawing 56,800 to a single show in Tampa, Florida, in 1973 and 120,000 to six concerts in the New York area in 1975. On paper at least, Led Zeppelin is unquestionably the world's most popular rock band.

Yes. But is it the world's best rock band?

That the question should even arise reflects not only this band's status, but also the current state of the music. What's the competition? The Rolling Stones. The Who. And?

Moreover, with the release of Physical Graffiti, Led Zeppelin's sixth album, the question has actually become relevant. This two-record set, the product of almost two years' labor, is the band's Tommy, Beggar's Banquet and Sgt. Pepper rolled into one: Physical Graffiti is Led Zeppelin's bid for artistic respectability.

In a virtual recapitulation of the group's career, Physical Graffiti touches all the bases. There's a blues ("In My Time of Dying") and a cosmic-cum-heavy ballad ("In the Light"); there's an acoustic interlude ("Bron-Y-Aur") and lots of bludgeoning hard rock, still this band's forte ("Houses of the Holy," "The Wanton Song"); there are also hints of Bo Diddley ("Custard Pie"), Burt Bacharach ("Down by the Seaside") and Kool and the Gang ("Trampled under Foot"). If nothing else, Physical Graffiti is a tour de force.

The album's — and the band's — mainspring is Jimmy Page, guitarist extraordinaire. It was Page who formed Led Zeppelin in 1968, after the model of such guitar-oriented blues-rock units as Cream, the Jeff Beck Group and the Yardbirds, where Page, a former sessionman, had first come to prominence. And it is Page who continues to chart Zeppelin's contemporary course, not only as the group's lead guitarist, but also as the band's producer.

His primary concern, both as producer and guitarist, is sound. His playing lacks the lyricism of Eric Clapton, the funk of Jimi Hendrix, the rhythmic flair of Peter Townshend; but of all the virtuoso guitarists of the Sixties, Page, along with Hendrix, has most expanded the instrument's sonic vocabulary.

He has always exhibited a studio musician's knack for functionalism. Unlike many of his peers, he rarely overplays, especially on record (and Led Zeppelin has never indulged itself with a live LP). Most of his playing instead evidences the restraint and rounded style of his avowed influences: the brooding, involuted blues lines of Otis Rush, the finely filigreed acoustic form of Bert Jansch, the echoed, subliminally driving accompaniments of Scotty Moore (behind Elvis Presley) and James Burton (behind Ricky Nelson) on early rockabilly records.

A facile soloist, Page excels at fills, obbligatos and tags. Playing off stock riffs, he modulates sonorities, developing momentum by modifying instrumental colors. To this end, he uses a wide array of effects, including on Physical Graffiti some echoed slide ("Time of Dying"), a countryish vibrato ("Seaside"), even a swimming, clear tone reminiscent of Lonnie Mack (the solo on "The Rover"). But his signature remains distortion. Avoiding "clean" timbres, Page usually pits fuzzed out overtones against a hugely recorded bottom, weaving his guitar in and out of the total mix, sometimes echoing Robert Plant's contorted screams, sometimes tunneling behind a dryly thudding drum.

Page's instrumental cohorts are John Paul Jones and John Bonham. Jones, another studio veteran, contributes keyboards as well as bass and is responsible, via his use of synthesizer, for bringing fullness as well as funk to the band. Bonham, on the other hand, is a steak-and-potatoes percussionist, handpicked, one assumes, for his ability to supply a plodding, stolid, rock-solid bottom — no one has ever accused Led Zeppelin of swinging.

Fronting the band onstage and sharing the spotlight with Page is vocalist Robert Plant. Like the Who's Roger Daltrey, he is a singer of limited range and feeling, but he projects himself with an irrepressible flair. Plant's acrobatics in fact complement Page's preoccupation with sound. Not only does Plant warble limply as well as scream, he also adds yet another gravelly component to the band. In his production of Plant, Page constantly plays on this grittiness, the vocal counterpart to the distorted sound of his own guitar.

Although Zeppelin at the outset hewed closely to the standard blues-rock format of the late Sixties, the band soon abandoned blues retreads to concentrate on their own brand of hard rock. The group's first album, Led Zeppelin, already contained such departures as "Dazed and Confused," a searing wall of sound that inspired a generation of heavy-metal rockers. "Communication Breakdown," also on the first LP, showed off the uptempo side of the Zeppelin format, with Page unleashing a blizzard of choppy chords. The jerky meter and crude attack remain favorite devices of Page, who, like Leiber and Stoller with the Coasters, understands the art of contriving a raucous sound (consider "Rock & Roll," Zeppelin's other masterpiece of distilled freneticism).

Thanks to Page's production, Led Zeppelin quickly outdistanced such predecessors as Cream and the Yardbirds. Not only was Plant a stronger singer than the Yardbirds' Keith Relf, but Page, in contrast to Clapton, Bruce and Baker, grasped the importance of crafting a coherent ensemble approach. Taking his cues from old Sun and Chess records, he used reverb and echo to mold the band into a unit, always accenting the bottom (bass and drums), always aiming at the biggest possible sound. As a result, Zeppelin's early records still sound powerful, while Cream tracks like "White Room" in retrospect sound pale and disjointed. On such classics as "Whole Lotta Love," Page's production set new standards for recording hard rock.

By 1971 and the release of the fourth Led Zeppelin album, Page and the band had broadened their approach to include acoustic ballads and folk-derived material, a side of the band introduced on Led Zeppelin III. "Stairway to Heaven," the band's most popular song, delicately balanced acoustic and electric elements before climaxing in a patented fuzz assault. Plant's controlled singing and Page's development of texture both distinguish this track, which to this day confounds critics who denigrate Zeppelin as a band schooled only in the art of excess.

But in fact an attention to detail and a sense of economy and nuance have become hallmarks of the Zeppelin style. "Four Sticks," from Led Zeppelin IV, to take a trifling example, sustains momentum by alternating a distorted electric riff with an acoustic progression doubled on keyboards. The percussion recalls Elvis's "Mystery Train" more than Cream's "Sunshine of Your Love," and it adds just the right touch of elegance to an otherwise elementary cut.

Physical Graffiti only confirms Led Zeppelin's preeminence among hard rockers. Although it contains no startling breakthroughs, it does afford an impressive overview of the band's skill. On "Houses of the Holy," Plant's lyrics mesh perfectly with Page's stuttering licks. Here again, the details are half the fun: Bonham kicks the cut along with a cowbell while the two final verses add what sounds like a squeaky chorus of "doit"s behind the vocal; Plant meanwhile is almost inaudibly overdubbed on the song's central chorus, underlining the phrase "let the music be your master."

Throughout the album, Page and the band tap a strange lot of sources, although the result is always pure Zeppelin. On "Ten Years Gone," a progression recalling the Beatles' "Dear Prudence" resolves in a beautifully waddling refrain, Page scooping broad and fuzzy chords behind Plant, who sounds a lot like Rod Stewart. Elsewhere, the band trundles out the Marrakech Symphony Orchestra (for "Kashmir"), Ian Stewart's piano and even a mandolin (both for "Boogie with Stu"). Small matter: Jimmy Page could probably arrange a quartet for finger cymbals and have it come lumbering out of the loudspeakers sounding like Led Zeppelin.

Naturally, Graffiti is not without faults — Zeppelin is too intuitive a band to cut a flawless album. Although Page and Bonham mount a bristling attack on "The Rover," this track, like several others, suffers from Plant's indefinite pitch. Other cuts, such as the ten-minute "Kashmir" and "In My Time of Dying," succumb to monotony. "In the Light," one of the album's most ambitious efforts, similarly fizzles down the home stretch, although the problem here is not tedium but a fragmentary composition that never quite jells: When Page on the final release plays an ascending run intended to sound majestic, the effect is more stilted than stately.

Despite such lapses, Physical Graffiti testifies to Page's taste and Led Zeppelin's versatility. Taken as a whole, it offers an astonishing variety of music, produced impeccably by Page. Not that this album will convince the doubters. Anyone with an antipathy to the posturing of Robert Plant or the wooden beat of John Bonham, be forewarned: A Led Zeppelin is a Led Zeppelin is a Led Zeppelin.

Physical Graffiti will likely also disappoint those who prefer their rock laced with lyrical significance: Led Zeppelin no more articulates a world view than Little Richard (or Cream) did. Yet while Zeppelin's stature as cultural spokesmen can be questioned, their standing as rock musicians cannot. True, Led Zeppelin misses the swagger of the Stones, the kinetics of the Who. But on Physical Graf